What plant disease led scientists to believe in the existence of viruses?

Th tobacco mosaic diease.

(T/F) Beijerinck was credited with being the first scientist to voice the concept as a virus, a particle much smaller and simpler than a bacterium.

True

The tiniest viruses are only 20 nm in diameter - smaller than a ribosome.

(T/F) In comparison to genes of other organisms, viruses have genomes that may consist of double-stranded DNA, single-stranded DNA, double-stranged RNA, or single-stranded RNA.

True.

It depends on the kind of virus. A virus is called a DNA virus or an RNA virus, according to the nucleic acid that makes up its genome.

The number of genes in their genomes vary from only four genes to several hundred to a thousand.

This is in comparison to bacterial genome that contain about 200 to a few thousand genes.

True.

What is the capsid?

The capsid is the protein shell enclosing the viral genome. Depending on the type of virus, the capsid may be rod-shaped, polyhedral, or more complex in shape like the T4 virus.

Viruses may also be contained within a membranous envelope.

What are the subproteins that are used to build capsids?

Capsomeres.

The number of different kinds of proteins in a capsid are usually smaller and are built from a large number of capsomeres.

Viruses with a rigid, rod-shaped capside arranged in a helix are typically known as ______________

Helical viruses

Describe the capsid of an adenovirus (the virus that typically infects the respiratory tracts of animals).

252 indeical protein molecules arranged in a polyhedral capsid with 20 triangular facets - an icosahedron. They are also referred to icosahedral viruses.

Some viruses have accessory structures that help them infect their hosts. For instance, a membranous envelope surrounds the capsids of influenza viruses and many other viruses found in animals.

The ___________, which are derived from the membranes of the host cell, contain host cell phospholipids and membrane proteins. They also contain proteins and glycoproteins of _________ origin.

Viral envelope, viral origin.

Many of the most complex capsides are found amount the viruses that infect _______________, known as ___________, or simply __________.

bacteria, bacteriophages, phages.

The most common example of a phage is the T4 virus.

Bacteriophages, or phages, consist of what components?

Elongated icosahedral heads enclosing the DNA and a protein tail piece with fibers by which the phages attach to a bacterium.

(T/F) Viruses reproduce in the same method as prokaryotes.

False.

Viruses lack the metabolic enzymes and equipment for making proteins, such as ribosomes. They are obligate intracellular parasites and can only reproduce only within a host cell.

It is fair to say that viruses in isolation are merely packaged sets of genes in transit from one host cell to another.

(T/F) Viruses sadly have the capability to infect any cell that it comes in contact with.

False.

Each type of virus can infect cells of only a limited variety of hosts, callled the host range of the virus.

As viruses are host specific, resulting from the evolutionary recognition systems developed over time, the "lock-and-key" fitting uses the viral surface proteins and _______ on the outside of the cell to be infected.

specific receptor molecules.

According to one model, such receptor molecules originally carried out functions that benefited the host cell but were co-opted later by viruses as portals of entry.

After a virus binds to a host cell and the viral genome makes its way inside, what two cycles may occur for the reproduction of phages?

How would one characterize the major difference between the cycles?

Lytic cycle and the lysogenic cycle.

The lytic cycle destroys the host cell whereas the lysogenic cycle allows for replication of the phage genome without destroying the cell.

Lytic cycle: Virulent or temperate phage, destruction of host DNA, production of new phages, Lysis of host cell causes release of progeny phages.

Lysogenic cycle: Temperate phage only, Genome integrates into bacterial chromosome as prophage which 1) is replicated and passed on to daughter cells and 2) can be induced to leave the chromosome and initate a lytic cycle.

What are the general steps taken by a virus for reproduction?

Virus binds to a host cell and the viral genome makes its way inside the host cell.

Once the viral genome is inside, the proteins it encodes can commandeer the host, repogramming the cell to copy the viral nucleic acid and manufacture viral proteins. The host provides the nucleotides for making viral nucleic acids and as well as enzymes, ribosomes, tRNAs, amino acids, ATP and other components needed for making the viral proteins.

After the viral nucleic acid molecules and capsomeres are produced, they spontaneously self-assemble into new viruses.

The reproductive cycle ends with the exit of the viruses.

What are the methods that viruses may allow for the viral genome to enter the host cell?

Phages - With the use of the elaborate tail apparatus, DNA is injected into a bacterium.

Enveloped viruses - Fusion of the viral envelope with the plasma membrane.

Some viruses may also enter the cell through endocytosis (the cell engulfs the virus).

As step 2 is occuring, host enzymes transcribe the viral mRNA, which host ribosomes use to make more capsid proteins.

What is occuring at 4 in a simplified viral reproductive cycle?

Viral genomes and capside proteins self-assemble into new virus particles, which exit the cell.

What is the lytic cycle?

A phage reproductive cycle that culminates in the death of the host cell. The term refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell.

A phage that reproduced only by a lytic cycle is a __________ phage.

Virulent phage.

Bacteria are not defenseless to phages. What are the three major reasons why this is so?

1) Natural selection favors bacterial mutants with receptors that are no longer recognized by a particular type of phage.

2) When phage DNA successfully eners a bacterium, the DNA is often identified as foreign and cut up by cellular enzymes called restriction enzymes (which restricts the ability of the phage to infect the bacterium)

3) Rather than lysing the host cells, many phages coexist with them in a state called lysogeny.

What are restriction enzymes?

The enzymes that restrict the ability of phages to infect bacterium. The bacterial cell's own DNA is methylated in a way that prevents attack by its own restrction enzymes.

However, natural selection favors phage mutants that can bind the altered receptors or are resistant to particular restriction enzymes.

What is the lysogenic cycle?

A type of phage reproductive cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host.

What are temperate phages?

Phages that are capable of using both modes of reproduction within a bacterium. A temperate phage is also called Lambda.

This is in contrast to virulent phages which are only capable of the lystic cycle.

What is occuring in step 1 of the lytic cycle of the virulent phage T4?

Attachment - The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell.

What is occuring in step 2 of the lytic cycle of the virulent phage T4?

Entry of phage DNA and degradation of host DNA - The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell's DNA is hydrolyzed.

What is occuring in step 3 of the lytic cycle of the virulent phage T4?

Synthesis of viral genomes and proteins - The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell.

What is occuring in step 4 of the lytic cycle of the virulent phage T4?

Assembly - Three separate sets of proteins self-assmble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms.

What is occuring in step 5 of the lytic cycle of the virulent phage T4?

Release - The phage directs production of an enzyme that damages the vacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles.

What is a prophage?

During alysogenic cycle, the lambda viral DNA molecule is incorporated into a specific site on the host cell's chromosome by viral proteins that break both circular DNA molecules and join them to each other. The viral DNA is known as a prophage.

One prophage gene codes for a protein that prevents transcription of most of the other prophage genes, thus the phage gnome is mostly silent within the bacterium and may be replicated to the daughter cells.

What does the term lysogenic imply?

It implies that prophages are capable of generating active phages that lyse their host cells. This occur when the lambda genome is induced to exit the bacterial chromosome and initiate a lytic cycle.

An encironmental signal, such as a certain chemical or high-energy radiation, usually triggers the switchover from the lysogenic to the lytic mode.

Should you look over the following figure each time you see this flash card?

Yes.

(T/F) Few bacteriophages have an envelope or RNA genome.

True.

However, in the case of animal viruses, some have both.

(T/F) Some animal virus have both an envelope or RNA gnome.

True.

However, in the case of bacteriophages, few have either.

For an animal virus equipped with an envelope, what are the purposes of the viral glycoproteins that protrude from the surface?

The viral glycoproteins bind to specific receptor molecules on the surface of a host cell.

What is occuring in step 1 of the reproductive cycle of an enveloped RNA virus?

Glycoproteins on the viral envelope bind to specific receptor molecules (not shown) on the host cell, promoting viral entry into the cell.

What is occuring in step 2 of the reproductive cycle of an enveloped RNA virus?

The capside and viral genome enter the cell. Digestion of the capside by cellular enzymes releases the viral genome.

What is occuring in step 3 of the reproductive cycle of an enveloped RNA virus?

The viral genome (red) functions as a template for synthesis of complementary RNA strands (pink) by viral enzyme.

What is occuring in step 4 of the reproductive cycle of an enveloped RNA virus?

New copies of viral genome RNA are made using complementary RNA strands as templates.

What is occuring in step 5 of the reproductive cycle of an enveloped RNA virus?

Complementary RNA stands also function as mRNA, which is translated into both capside proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER and Golgi apparatus)

What is occuring in step 6 of the reproductive cycle of an enveloped RNA virus?

Vesicles transport envelope glycoproteins to the plasma membrane.

What is occuring in step 7 of the reproductive cycle of an enveloped RNA virus?

A capside assmbles around each viral genome molecule

What is occuring in step 8 of the reproductive cycle of an enveloped RNA virus?

Each new virus buds from the cell, its envelope studded with viral glycoproteins embedded in membrane derived from the host cell.

Among the three types of single-stranded RNA genomes found in animal viruses, the genome of class IV viruses

a) can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
b) serves as template for mRNA synthesis. The RNA genome is transcribed into complementary RNA strands which function both as mRNA and as templates for the synthesis of additional copies of genomic RNA. (RNA to RNA synthesis)
c) are known as retroviruses and areequipped with an enzyme called reverse transcriptase which transcribes and RNA template into DNA, providing an RNA to DNA information flow, the opposite of the usual direction.

A

Among the three types of single-stranded RNA genomes found in animal viruses, the genome of class V viruses

a) can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
b) serves as template for mRNA synthesis. The RNA genome is transcribed
into complementary RNA strands which function both as mRNA and as
templates for the synthesis of additional copies of genomic RNA. (RNA to
RNA synthesis)
c) are known as retroviruses and areequipped with an enzyme called reverse transcriptase which transcribes and RNA template into DNA, providing an RNA to DNA information flow, the opposite of the usual direction.

B

Among the three types of single-stranded RNA genomes found in animal viruses, the genome of class VI viruses

a) can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
b) serves as template for mRNA synthesis. The RNA genome is transcribed
into complementary RNA strands which function both as mRNA and as templates for the synthesis of additional copies of genomic RNA. (RNA to RNA synthesis)
c) are known as retroviruses and areequipped with an enzyme called reverse transcriptase which transcribes and RNA template into DNA, providing an RNA to DNA information flow, the opposite of the usual direction.

C

Describe what is special about a retrovirus (class VI).

Retroviruses are class VI viruses that are equipped with a reverse transcriptase enzyme that transcribes an RNA template into DNA, provide an RNA to DNA information flow, the opposite of the usual direction.

The unusual phenomenon is the source of the name retroviruses (retro means "backward")

HIV (__________________) is the retrovirus that causes AIDS (_________________). HIV and other retroviruses are enveloped viruses that contain two identical molecules of __________-stranded RNA and two molecules of _____________.

Human immunodeficiency virus

acquired immunodeficiency syndrome

single-stranded RNA

reverse transcriptase

What is a provirus?

An integrated viral DNA, that never leaves the host's genome and remains a permanent resident of the cell. The integration of a provirus occurs in retroviruses.

This is in contrast to a prophage that leaves the host's genome at the start of the lytic cycle.

(T/F) Viruses depend on cells for their own propogation, it seems likely that they proceeded precellular forms of life.

False.

This statement makes no sense as viruses are not considered living and it is most likely that viruses evolved after the first cells appeared. They have been found to infect every form of life.

Candidate for the origional sources of viral genomes include ________ and ________. They are both mobile genetic elements.

Plasmids - small, circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts. Plasmids exist apart from the cell's genome, and can replicate independently of the genome. They are occasionally transferred between cells.

Transposons - DNA segments that can move from one location to another within a cell's genome.

What is a vaccine?

A harmless variant or derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen.

What three processes contribute to the emergence of viral diseases?

1) Mutation of existing viruses. RNA viruses tend to have an unusually high rate of mutation because errors in replicating their RNA genomes are not corrected by proofreading.

2) Dissemination of a viral disease from a small isolated human population.

3) Spread of existing viruses from other animals.

What is a flu epidemic?

General outbreaks of flu.

What is a pandemic?

A global epidemic or global outbreak of a flu.

Which of the three types of influenza viruses cause epidemics by infecting a wide range of animals including birds, bigs, horses, and humans?

Type A.

Type B and C only infect humans and have never caused an epidemic.

What are viroids? What do they infect?

Circular RNA molecules, only a few hundred nucleotides long, that infect plants. Viroids do not encode proteins but can replicate in host plant cells, apparently using host cell enzymes. They also disrupt plant growth.

An important lesson from viroids is that a single molecule can be an infectious agent that spreads a disease.

What is a prion? What do they infect?

An infectious agent that is a misfolded version of a normal cellular protein. Prions appear to increase in number by converting correctly folded versions of the protein to more prions.

They tend to cause brain diseases in mammals.

What two main factors about prions are of large concern?

Prions are infectious proteins. An infectious agent that is a misfolded version of a normal cellular protein. Prions appear to increase in number by converting correctly folded versions of the protein to more prions.

1) prions act very slowly, with an incubation period of at least ten years before symptoms develp. The lengthy incubation period prevents sources of infection from being identified until long after the first cases appear, allow many more infections to occur.

2) Prions are virually indestructible; they are not destroyed or deactivated by heating to normal cooking temperatures.

To date, there is no known cure to prion diseases.

Contrast horizontal and vertical transmission of viruses in plants.

In horizontal transmission - A plant is infected from an external source of virus, which could enter through a break in the plant's epidermis due to damage by herbivores.

Mutations can lead to a new strain of a virus that can no longer be effectively fought by the immune system, even if an animal had been exposed to the original strain, a virus can jump from one species to a new host; and a rare virus can spread if a hos population becomes less isolated.

TMV has been isolated from virutally all commercial tobacco products. Why, then, is TMV infection not an additional hazard for smokers?

Humans are not in the host range of TMV, so they can't be infected by the virus.

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phages protein coat and T4 phage DNA. The new phages produced would have
a) T2 protein and T4 DNA
b) T2 protein and T2 DNA
c) a mixture of the DNA and proteins of both phages.
d) T4 protein and T4 DNA
e) T4 protein and T2 DNA

Which of the following characteristics, structures, or processes is common to both bacteria and viruses?
a) metabolism
b) ribosomes
c) genetic material composed of nucleic acid
d) cell division
e) independent existence

C

Emerging viruses arise by
a) mutation of existing viruses
b) the spread of existing viruses to new host species
c) the spread of existing viruses more widely within their host species.
d) all of the above.
e) none of the above.

D

To cause a human pandemic, the H5N1 avian flu virus would have to
a) spread to primates such as chimpanzees.
b) develop into a virus with a different host range.
c) become capable of human to human transmission
d) arise independently in chickens in North and South America.
e) become much more pathogenic.